Absorbent film produced by vacuum evaporation

ABSTRACT

Absorbent film which has a great adhesion, a great mechanical strength and a high resistivity to heat and other radiation and which is applicable to sun glasses, color filters, phase plates used for the differential phase method, apodization filters and the like. The absorbent film is produced by mixing together powdered Ti, powdered Cr and powdered MgF 2  into the form of a powdered mixture or shaping them into a compressed mass of mixture, thereafter vacuum-evaporating said mixture onto a predetermined base material such as lens glass to thereby provide a refractive factor of about 1.52 and an absorption coefficient of about 0.01 to 0.4. At least one layer of such absorbent film may be used to form a multi-layer film structure.

This application is a continuation-in-part of my copending applicationSer. No. 117,288, filed Feb. 22, 1971, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to absorbent film provided by a vacuumevaporation technique.

2. Description of the Prior Art

It has widely been practiced to produce absorbent film by the use of thevacuum evaporation technique and employing such film for color filters,neutral filters, sun glasses and phase plates for the differential phasetechnique.

Such absorbent film is usually composed of a metal or a metal compoundespecially such as metal oxide, and has a high refractive factor and ahigh absorption coefficient, which will not only result in a highreflection factor in both sides of the film adjacent the atmosphere andthe base glass but also lead to a less aesthetic appearance and a greatinconvenience in use.

In order to reduce the amount of reflected light, a single or multiplelayers of dielectric film have been formed on the absorbent film byusing the vacuum evaporation technique so as to prevent the occurrenceof light reflection. Even this method, however, has encountereddifficulties in effectively preventing the reflection of visible raysover their entire range, and the reflected light in this case is alwaysseriously colored. To attain a predetermined coloring of such reflectedlight, the layer or layers of dielectric film to be formed on theabsorbent film must be of a very high accuracy and otherwise controlled,and this makes it difficult to achieve a reproductivity of such film. Inaddition, presence of any slight fingerprint, water drop, fatty orgreasy material or the like on the film would immediately vary thecoloring of light to seriously injure the aesthetic value of the film.

Evidently, these drawbacks may be avoided by providing a thick anduniform film of a low absorption coefficient and whose refractive factorfor the entire range of visible light is about 1.52, which isapproximately equal to that of the conventionally used base glass. Thisis because such a film formed on the glass surface would permit onlyabout 4% of the incident light to be reflected and thereby eliminate anycoloring. The formation of such film may be accomplished by any of thevarious methods shown below.

I. A mixture of a dielectric material (non-absorbent material) and ametal (absorbent material) is evaporated from a single source ofevaporation;

ii. The same mixture is evaporated from two discrete sources ofevaporation; or

iii. A non-uniform film is produced so that the density ratio of theabsorbent material to the non-absorbent material is least in theopposite surfaces of the film adjacent to the base glass and theatmosphere and greatest in the intermediate region of the film. Therefractive factor in the said opposite surfaces of the film issubstantially equal to the refractive factor of the base glass.

Success of the first-named method above is very much dependent on thematerials selected, and usually it is very difficult to form a uniformand sufficiently thick film of a mixture at a predetermined ratiothrough evaporation. Even if such film could be attained at all, anythickness thereof exceeding a certain value would cause the film toreadily separate from the base glass (weak adhesion) or to be fractured(inferior mechanical strength).

In some instances such film is poor in the resistivity to abrasion, heatand humidity, and in many instances it becomes seriously brownish due tothe oxidation resulting from ultraviolet rays. A film having a highresistivity to mechanical, chemical and weather conditions could beformed by this method, but this would be impossible without resorting tothe electronic beam evaporation technique or the like.

The second-named method above is unsuitable for mass production becauseit involves very delicate control of the rates of evaporation from thetwo sources of evaporation and some other delicate controls in order toprovide a uniform film of a predetermined mixing ratio.

The third-named method is also difficult in terms of reproductivitybecause considerably complex and sophisticated controls are involvedtherein as in the case of the second method.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide absorbent filmhaving a great adhesion, a great mechanical strength and a highresistivity to heat and other radiation by using the principle of themethod mentioned under item i) above while employing magnesium fluorideas the dielectric material and powdered titanium and chromium as themetal material.

It is another object of the present invention to provide a multi-layerfilm structure which comprises at least one layer of such absorbent filmas described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from the followingdescription of some specific embodiments thereof taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is an enlarged cross-sectional view of a meniscus lens of theprior art showing the reflective effect from the concave side thereof;

FIG. 2 is an enlarged cross-sectional view of the lens of FIG. 1 showingthe reflective effect from the convex side thereof;

FIG. 3 is an enlarged cross-sectional view of the lens of FIG. 1 showingthe reflective effect of a finger print on the surface of the film;

FIG. 4 is an enlarged cross-sectional view of a meniscus lens of thepresent invention showing the reflective effect from the concave sidethereof;

FIG. 5 is an enlarged cross-sectional view of the lens of FIG. 4 showingthe reflective effect from the convex side thereof;

FIG. 6 is an enlarged cross-sectional view of the lens of FIG. 4 showingthe reflective effect of a finger print on the surface of the film;

FIGS. 7(a) and 7(b) are enlarged cross-sectional views of a concavemeniscus lens and a convex meniscus lens each having an absorbent filmof the present invention applied thereto; and

FIGS. 8(a), 8(b) and 8(c) are graphs illustrating the spectraltransmittivity characteristics of the absorbent film embodying thepresent invention, the spectral reflection characteristics of such filmin the surface thereof adjacent to the atmosphere, and the spectralreflection characteristics in the other surface adjacent to the baseglass, respectively.

DETAILED DESCRIPTION

The prior art glass shown in FIG. 1 is coated with an absorbent thinfilm F₁ on the concave surface to attain the predeterminedtransmissivity. The prior art glass is further coated with dielectricanti-reflection film F₂ on the absorbent film F₁ so as to reducereflection at the outer surface of the absorbent film F₁, or otherwisethe reflection would be increased by 20-30%. In the prior art glass,refractive index n₁ of the absorbent film F₁, refractive index n₂ of theanti-reflection film F₂ and refractive index n of the glass G aredifferent from each other so that optical interfaces S₄ and S₂ areformed between the glass G and the film F₁ and between the films F₁ andF₂. An observer would see colored reflection light, for instance bluishreflection light, when he looks at the prior art glass from the rightside (II), as shown in FIG. 1. The colored reflection light is caused bythe interferences of the reflection light rays R₁ at the surface S₁, thereflection light rays R₁ ' on the optical interface S₂ and thereflection light rays R₁ " on the optical interface S₄. Such a coloredreflection light caused by the interference of the reflection light makea bad appearance.

It is a first object of the present invention to eliminate undesirablecolors caused by the interference of the reflection light rays at theair side of said thin films, and to thereby make a good appearance.

When the prior art glass is viewed from the convex side thereof, asshown by arrow I in FIG. 2, one can see, two kinds of lights, coloredand white reflection lights, for example. The incident light rays L₂ arefirstly reflected at a surface S₃ of the glass G to make reflectionlight rays R₂ '" which are acknowledged as the white color. The incidentlight rays L₂ which are not reflected at the surface S₃, are thenreflected at the optical interfaces S₄, S₂ and the surface S₁. Thecolored reflection light is caused by the interferences of thereflection light rays R₂ " on the optical interface S₄, the reflectionlight rays R₂ ' on the optical interface S₂ and the reflection lightrays R₂ at the surface S₁.

Such colors, when the glass is observed from the direction shown by thearrow I, make an undesirable appearance.

A second object of this invention is to prevent the convex sidereflection light from being colored by interference, so that the convexside reflection light is similar to the transmission light, therebymaking a good appearance.

In the event that a finger print O is left on the surface S₁ of the filmF₂, as shown in FIG. 3, one can see a luminous image of the finger printO, the image having different color and brightness from the interferencecolors of the other portions, when the lens is viewed from the directionof arrow II. The incident light rays L₃ are partially reflected, firstlyat the surface S₅ of the finger print O as reflection light rays R₃,secondly on the optical interface S₁ between the finger print O and thefilm F₂ as reflection light rays R₃ ', thirdly at the optical interfaceS₂ as reflection light rays R₃ ", and at the optical interface S₄ asreflection light rays R₃ '". By the interference of these reflectionlight rays R₃, R₃ ', R₃ " and R₃ '", there is formed a luminous fingerprint image.

It is a third object of the invention to prevent the thin filminterference when any oil, or the like, is left on the surface of thethin film so as to eliminate the luminous reflection light.

The said first, second and third objects of the present invention can beattained by approximately equalizing the refractive index n' of anabsorbent film F with the refractive index n of a glass G so that theoptical interface between the glass and the absorbent film iseliminated.

Thus, incident light rays L₄ are not reflected at the surface T₂ betweenthe glass G and the absorbent film F, but are reflected at the surfaceT₁, as shown in FIG. 4. If the subject glass is viewed from thedirection of arrow II, the reflected light rays R₄ do not interfere sothat only white color is visible, which is the same as that reflected atthe glass surface. Thus, said first object is attained.

In the glass G, there is practically no optical interface, and so thereis no reflection at the surface T₂, as shown in FIG. 5. Therefore, thereflected light rays R₅ at the side of the glass can not interfere, sothat there can be no coloration of the reflection light rays at theglass side of the film F. If the glass is viewed from the direction ofarrow I, as shown in FIG. 5, one will observe colored reflection lightrays, which are similar to the transmitting light rays. Thus, the saidsecond object can be attained.

As best seen in FIG. 6, the refractive index of a finger print Oapproximately equals that of the glass G so that there is no opticalinterface between the absorbent film F and the finger print O.Accordingly, the light rays L₆ are not reflected at the surface T₁between the finger print O and the absorbent film F, but are reflectedonly at the surface T₄ of the finger print O, as shown in FIG. 6. Thus,there is no luminous light from the surface of the finger print O, andconsequently the third object of the present invention is attained.

The fourth object of the present invention is attained by substantiallyequalizing the refractive index of the thin film F with that of theglass and by reducing the absorbent coefficient of the thin film Fwithin 0.01-0.4. It is noted that as a practical matter it is difficultto equalize the refractive index of the absorbent film F with that ofthe glass. However, if the thickness of the thin film is at least morethan one wave-length, then no interference occurs even if the refractiveindices are slightly different from each other.

For attaining the aforesaid first to fourth objects while forming anabsorbent thin film having the desired transmissivity, it is necessaryto vary the absorbent coefficient in accordance therewith. For thispurpose, the weight ratio among the vapor-deposition materials Cr, Tiand MgF₂ is varied. For forming an absorbent thin film having theabsorbent coefficient ranging from 0.4 to 0.01, the weight ratio of thevapor-deposition materials is determined to be Cr: 1.0, Ti: 0.02-0.2 andMgF₂ : 1.5-0.10, for example.

Referring to FIGS. 7(a) and 7(b), there are shown examples of theabsorbent film applied to two different types of lens according to thepresent invention. The present invention in principle falls within thecategory of the method referred to under item i) above, and employsmagnesium fluoride (MgF₂, n = 1.40) as the dielectric material, andpowdered titanium (Ti) and chromium (Cr) as the metal material. Thesethree materials at a suitable ratio are sufficiently mixed togethermechanically so as to form a powdered mixture or further shaped into acompressed mass. The mixture is then evaporated from a single source ofevaporation by resistance-heating it in a vacuum of 10.sup.⁻⁵ Torr.

An evaporated film thus obtained has a low absorption coefficient and alow refractive index which is at best n≈1.5. The evaporating conditionof the mixture is maintained very stably in the vacuum of 10.sup.⁻⁵Torr, and the result is a uniform and sufficiently thick absorbent filmhaving such an excellent reproductivity that the naked eye would senseno appreciable difference either for the color of the transmitted lightor for the color of the reflected light. If the glass on which theabsorbent film is evaporated has a refractive index of about 1.52, areflection index of 4% may be provided anywhere in the surface of thefilm adjacent the atmosphere. The optical density of the evaporated filmis determined only by the thickness thereof, and the maximum possibledensity is D≈1.0 or greater for λ = 5000A. Irrespective of the high orlow temperature in the base glass, the evaporated film thus provided hasa greater strength than a film formed by evaporating a single componentmaterial.

If the base glass is heated to about 350°C, the resulting product willnot suffer from any variation due to aging and will have a greatlyincreased wear resistivity and adhesion and, also, a practicallysufficient resistivity to chemicals. The test of resistivity to weather,particularly to heat (8-hour exposure to air at 300°C) and exposure toultraviolet rays (200-hour exposure to ultraviolet rays ten times themean sunlight) has showed that the variation in the reflection index islower than ± 0.2% for both surfaces of the film which are adjacent tothe atmosphere and the base glass, respectively.

The combination of the aforesaid two materials MgF₂ and Cr is selectedon the grounds described hereinafter.

The evaporating temperatures of MgF₂ and Cr for a vacuum of 10.sup.⁻⁴Torr are 1540°C and 1430°C respectively, which may be regarded asapproximately equal to each other. This is very useful for thecombination.

Further, even if the evaporation of such mixture follows Raoult's laws(see the table below), the values of P/√M for MgF₂ and Cr areapproximately equal to each other so that a stable evaporation can beexpected to occur.

It has also been found empirically that the addition of powdered Ti as athird component to MgF₂ and Cr greatly increases the adhesion,mechanical strength and resistivity of the resulting film to radiationssuch as heat and ultraviolet rays.

    ______________________________________                                        Molecular   Evaporation Pressure(P)                                           Weight      at 1400°C (T)                                                                            P/√MmmHg                                 ______________________________________                                        MgF.sub.2                                                                           62.3      1 × 10.sup..sup.-5 mmHg                                                                   1 × 10.sup..sup.-6                    Cr    52.0      5 × 10.sup..sup.-5 mmHg                                                                   7 × 10.sup..sup.-6                    Ti    47.9      2 × 10.sup..sup.-7 mmHg                                                                   3 × 10.sup..sup.-8                    ______________________________________                                    

The combination of these three materials ensures a stable evaporationfor a very wide range of mixing ratios, without causing the materials tovaporize and evaporate discretely from one another, and the mixing ratioof the three materials can be empirically selected to an optimum valuein accordance with such factors as the spectral transmittivity orspectral density to be obtained, the desired spectral reflection factor,the color variation of the reflected light resulting from the variationof the film thickness, the temperature of the glass base, and the sizeof the evaporation boat in use.

The ratio in weight between the three materials is shown below by way ofexample.

    ______________________________________                                                 Ti          0.02 - 0.2                                                        Cr          1.0                                                               MgF.sub.2   0.10 - 1.5                                               ______________________________________                                    

The boat used for the resistance-heating should be formed by molybdenumand have a sufficient current capacity and a sufficient volume capacity.The boat is charged with a suitable quantity of the mixture to beevaporated and then subjected to breathing and preheating, whereafterthe mixture is evaporated while being maintained at a temperature ofabout 1100°C by a predetermined heating source. Thus, there is provideda more stable condition for vaporization and evaporation.

Referring to FIGS. 8(a), 8(b), and 8(c), there are shown the spectraltransmittivity characteristics (T) and the spectral reflective indexcharacteristics (R for the surface adjacent to the atmosphere and R' forthe surface adjacent the glass) of the single-layer film with respect tothe base glass (whose refractive index is 1.52) in an embodiment of thepresent invention. In FIGS. 8(a),, 8(b) and 8(c), the solid-line curves,dashed-line curves and dotted-line curves, respectively, represent themeasurements of the three types of films having different opticaldensities. As will be seen from the spectral reflective characteristicsshown in FIGS. 8(a), 8(b) and 8(c), the reflected light R in the surfaceof the film exposed to the atmosphere is white or non-colored, while thereflected light R' in the other surface adjacent to the base glass issimilar in color to the transmitted light. Therefore, the appearance ofthe lens can be greatly improved by disposing the latter surface so asto look toward the atmosphere.

The lens thus provided creates no coloring in the reflected light evenif it is spotted with fingerprints, water drops or greasy stains, and inaddition, it has an improved reproductivity. Moreover the single-layerstructure of the absorbent film only requires that care be taken of thetransmittivity to monochromatic light during the manufacture, and thisleads to simplified mass production of absorbent films having anydesired density.

Further, it is of course possible to additionally provide a reflectionpreventing film of the known type on the described single-layer filmformed according to the present invention.

The absorbent film produced according to the present invention will findvarious applications in the following fields:

1. Sun glasses having a surface adjacent to the atmosphere forpermitting white light to be reflected at a low rate of reflection, anda surface adjacent to the glass for permitting the reflection of lightsimilar in color to the transmitted light, both reflected lights beingsubstantially unaffected by any stain or spot present in those surfaces.

2. Color filters with an evaporated film having an especially lowreflection surface adjacent to the glass and a surface adjacent to theatmosphere provided with a reflection characteristic similar to that ofa solid filter.

3. Absorbent phase plates of a low reflection index used for the phasedifference method. For this purpose, the prior art has employed ametallic film which tends to produce a great quantity of harmfulreflected light and requires an additional film for preventing suchreflection.

4. Absorbent filters such as apodization filters capable of transmittingphase information as well. Since the absorbent film of the presentinvention can be made to have a refractive index of about 1.52, thematching of the refractive index can be accomplished readily andaccurately by the use of cement such as balsam or the like.

5. Since the absorbent film of the present invention can be regarded asan approximate equivalent to the base glass whose refractive index isabout 1.52, any such film in a single layer or multiple layers designedso as to suit an overlying or underlying glass base having a refractiveindex of about 1.52 can be applied thereto substantially withoutchanging or modifying it in any way. In fact, the absorbent film of thepresent invention may be applied by evaporation to either side of areflection preventing film, a reflection promoting film, a band-passfilter such as cold filter, an interference filter or the like so as toprovide them with absorbent characteristics, as well as increasedadhesion or protection effect.

6. The absorbent film of the present invention means a material havingany low absorption coefficient in the following range of opticalconstants:

n (refractive index) ≈ 1.45- 1.60

k (absorption coefficient) ≈ 0.01- 0.4

Thus, the present invention enables an unknown or novel material havinga low absorption coefficient to be produced artificially, as desired,and it can be utilized to design a film structure including at least onelayer of film having such optical constants or to realize a design whichwill require the use of at least one layer of such film.

Although certain particular embodiments of the invention are hereindisclosed for purposes of explanation, various modifications thereof,after study of this specification, will be apparent to those skilled inthe art to which the invention pertains.

What is claimed and desired to be secured by Letters Patent is:
 1. In anabsorbent film deposited on a substrate having a refractive index ofabout 1.52 by evaporating a mixture of plural materials within a vacuum;the improvement wherein the mixture comprises Cr, Ti and MgF₂ and theratio in weight of said Cr, Ti and MgF₂ is 1.0 : 0.02 to 0.2 : 0.10 to1.5 such that the absorbent film has the same refractive index as thesubstrate and an absorption coefficient of from about 0.01 to about 0.4.2. An absorbent film as defined in claim 1, in which said absorbent filmis deposited on the substrate by evaporation in a vacuum of 1 to 10 ×10.sup.⁻⁵ Torr.